When performing a snapshot using the SMAPI, set is_a_snapshot

This is usually fixed up after the fact, either in xapi_vdi.ml
in the case of a normal VDI snapshot, or in update_snapshot_info_dest.
If the snapshot_of information is missing though, this does not
get set.

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>